Optical recording medium and optical information processor

ABSTRACT

An optical recording medium (D) includes tracks (T) extending along a spiral. The tracks (T) include a plurality of first sections (Ta) and a plurality of second sections (Tb) differing from the first sections with respect to at least one of width and depth. Each of the first sections (Ta) includes a first overlapping portion ( 11 ) in which the trailing end and other portion of the first section overlap each other in the tracking direction, the trailing end and the other portion of the first section being located in adjacent tracks. Each of the second sections (Tb) includes a similar configured second overlapping portion ( 12 ). Due to such a structure, the beam spot can be easily caused to circulate on the same track of the optical recording medium (D) during idling.

TECHNICAL FIELD

The present invention relates to an optical recording medium and anoptical information processing apparatus. The term “optical recordingmedium” as used herein refers to a recording medium whereby data can berecorded and/or reproduced using optical means, and is a general termencompassing not only strictly optical recording medium such as CD-ROMbut also recording media whereby data can be rewritten by means of amagneto-optical recording system, phase change system, or the like.

BACKGROUND ART

An example of an optical disk is shown in FIG. 7. This disk is providedwith a recording layer in which lands L and grooves G are formed in analternating fashion along the tracking direction Tg (radial direction ofthe optical disk). A push-pull method, for example, is available as amethod for performing tracking control in an optical disk device thatuses such an optical disk. The interference of light reflected theoptical disk is indispensable for understanding the present inventionwhich utilizes this method, and is therefore described below.

As shown in the same figure, when a beam of light is projected to thegrooves G of the optical disk, the reflected light includes a bundle ofzero-order light rays R₀ and two bundles of first-order light rays R₁interfering with each other to generate interference light Ia. Thezero-order light R₀ consists of non-diffracted light that is reflectedso as to follow the irradiating path of the light beam toward thegrooves G. In contrast, the two bundles of first-order light rays R₁consist of positive and negative first-order diffracted light rays thatare generated due to the fact that the lands L and the grooves G arearranged in side-by-side relationship in the tracking direction. Whenthe beam of light is irradiated at a position offset from the center ofthe groove G in the tracking direction, the two bundles of first-orderlight rays R1 become asymmetric and the reflected light developsdifferential intensity in the tracking direction. The reflected light isdetected using an optical detector 9 having two light-receiving sections90 e, 90 f arranged in the tracking direction Tg, as shown in FIG. 8 forexample. Electrical signals Se, Sf each having a level corresponding tothe amount of received light (light intensity) by the light-receivingsections 90 e, 90 f are outputted from the optical detector 9, and atracking error signal (push-pull signal) is created by taking thedifference of the two electrical signals. This tracking error signalrepresents the direction and amount of the tracking error.

The track pitch need be reduced with a resulting increase of the datarecording density in order to increase the data recording capacity ofthe optical disk. However, as the track pitch is reduced, the intervalbetween the two bundles of first-order light rays R₁ widens, so that theproportion of interference light Ia contained in the reflected lightreduces. Eventually leads, a so-called diffraction limit is reached inwhich no interference light Ia is contained in the reflected light. Thelimit track pitch Pt that causes the diffraction limit is theoreticallydefined as Pt=λ/NA/2 (where λ is the wavelength of the light beam and NAis the numerical aperture of the objective lens). It becomes difficultto detect a tracking error by the above-mentioned method when the trackpitch of the optical disk is smaller than the limit track pitchdescribed above.

A conventional counter-measure is disclosed in JP-A 2000-331383.According to the conventional counter-measure, grooves Gd, Gt adjacentto each other in the tracking direction Tg are designed to differ fromeach other with respect to one or both of the depth and width, as shownin FIG. 9. The grooves Gd, Gt are formed to extend along twoco-extensive spirals as shown in FIG. 10A, or to extend along a singlespiral in which they are alternately connected to each other as shown inFIG. 10B.

According to this prior art technique, the apparent spatial frequency ofthe optical disk can be made ½. Consequently, a tracking error can bedetected even when the track pitch is made narrower than the limit trackpitch established by the equation Pt=λ/NA/2 above.

However, the prior art technique described above has the followingdrawbacks. In the former structure shown in FIG. 10A, the two groovesGd, Gt extend along two parallel spirals, so when data are continuouslywritten in the tracks of the optical disk, for example, processing forwriting to the groove Gd and processing for writing to the groove Gtmust be alternately performed. The control of continuous writing whilechanging the writing-target tracks in this manner is not easy, and itsimplementation is difficult. In order to form the grooves Gd, Gt by adepicting method that uses an electron beam in the manufacture of theoptical disk, two electron beams must be used, so the optical disk isalso difficult to manufacture.

In contrast, in the latter structure shown in FIG. 10B, the grooves Gd,Gt are formed along a single spiral, so the grooves can be formed usinga single electron beam, and the groove forming operation is facilitated.Write processing is also facilitated because there is no need to changethe writing-target tracks when data are continuously written in thetracks. However, the latter structure still has the following drawbacks.

Firstly, it is not easy to perform control whereby the optical head isheld facing the same track during idle time in which no data are writtenor read. The reason for this is that because the tracks extend along aspiral, the optical head cannot be held facing the same orbital tracksolely by performing tracking control on the basis of the tracking errorsignal obtained by the above-described method, and control for trackjumps, referred to as “track jumping”, must be performed.

Secondly, both grooves Gd, Gt are equally interrupted at the borderlineindicated by the line L1 shown in FIG. 10B. Therefore, a sudden changeoccurs whereby the polarity of the tracking error signal obtained atthat time is reversed when the beam of light continues to irradiate thesection near the changeover point. Consequently, when the optical headattempts to find the number of traversed tracks by counting the numberof zero points in the tracking error signal during a seek operation formoving the optical head to a position opposite the target track,counting errors can easily occur due to erroneous counting of the suddenchange in the tracking error signal described above.

Thirdly, the sensitivity with which the tracking error is detected is byno means high, and it is difficult to perform highly accurate trackingcontrol on the basis of the tracking error signal. This third drawbackis also encountered in the former structure shown in FIG. 10A.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an optical recordingmedium and an optical information processing apparatus that are capableof overcoming or alleviating the foregoing drawbacks.

According to a first aspect of the present invention, there is providedan optical recording medium comprising:

a recording layer having a surface which is indented in cross sectionextending in a tracking direction, said surface being formed with tracksextending along a spiral;

the tracks including a plurality of first sections and a plurality ofsecond sections, the first sections differing from the second sectionswith respect to at least one of width and depth, the first sectionsalternating with the second sections in a manner such that a trailingend of one section is connected to a leading end of another section;

a first overlapping portion in which the trailing end and anotherportion of each first section overlap each other in the trackingdirection, said trailing end and said another portion of said each firstsection being located in adjacent tracks; and

a second overlapping portion in which the trailing end and anotherportion of each second section overlap each other in the trackingdirection, said trailing end and said another portion of said eachsecond section being located in adjacent tracks.

Preferably, the recording layer comprises grooves and lands alternatingwith the grooves in the tracking direction.

Preferably, the optical recording medium is a groove-recording disk forwriting data in the grooves.

Preferably, the optical recording medium is a read-only disk wherein thetracks are formed with a plurality of pits for data.

Preferably, at least one shift point between the first section and thesecond section is provided in every turn of the tracks.

Preferably, the optical recording medium further comprises a pluralityof gaps for creating a thicknesswise height difference in the tracks,and the gaps positioned in each track are offset from the gapspositioned in an adjacent track in the track direction.

Preferably, the gaps are formed by spacing the grooves from each otherin the track direction.

Preferably, the gaps are arranged at a constant pitch at least in a sametrack.

Preferably, the amount of the offset in the track direction between thegaps in adjacent tracks is set at 1/N (where N is an integer of no lessthan 2) of the gap pitch in one of the tracks.

According to a second aspect of the present invention, there is providedan optical information processing apparatus comprising:

an optical head disposed in facing relationship to an optical recordingmedium for emitting a beam of light to the optical recording medium andfor receiving the light reflected from the optical recording medium;

an optical detector for receiving the reflected light from the opticalrecording medium via the optical head to output a signal correspondingto an intensity distribution of the reflected light; and

a push-pull signal generator for generating a radial push-pull signaland a tangential push-pull signal in dependence on an intensitydifference of the reflected light in a tracking direction and in a trackdirection on the basis of the signal outputted from the opticaldetector;

a first signal processor for generating a first tracking error signal byremoving a noise component from the radial push-pull signal;

a second signal processor for generating a second tracking error signalby sampling and holding the radial push-pull signal when a predeterminedchange has occurred in the tangential push-pull signal; and

a controller for performing tracking control on the basis of the firsttracking error signal during idle time in which data are neither writtenin nor read from the optical recording medium while performing trackingcontrol on the basis of the second tracking error signal when data arewritten or read.

Preferably, the first signal processor is a filter for removing ahigh-frequency component of no less than a predetermined minimumfrequency.

Preferably, the optical information processing apparatus furthercomprises a track counter for counting the tracks during a seekoperation by detecting and counting zero points of the first trackingerror signal.

Preferably, the second signal processor comprises a detection circuitfor detecting amplitude and zero point of the tangential push-pullsignal, and a sample hold circuit for sampling the radial push-pullsignal when a predetermined amplitude and the zero point are detected bythe detection circuit.

Preferably, the optical information processing apparatus furthercomprises a clock signal generator for generating a clock signal insynchronization with a timing at which the zero point is detected by thedetection circuit.

Other features and advantages of the present invention will becomeclearer from the following description of embodiments of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view showing an example of a magneto-opticaldisk to which the present invention is applied.

FIG. 2 is a plan view illustrating the main structure of themagneto-optical disk in FIG. 1.

FIG. 3 is a diagram showing the relationship between the gaps andgrooves of the magneto-optical disk illustrated in FIG. 1.

FIG. 4 is a schematic block diagram showing an example of amagneto-optical disk apparatus to which the present invention isapplied.

FIG. 5A is a diagram showing a specific example of a TPP signal; FIG. 5Bis a diagram showing a specific example of a RPP signal; and FIG. 5C isa diagram showing the conditions for obtaining the TPP signal and theRPP signal.

FIG. 6 is a schematic diagram showing another example of the presentinvention.

FIG. 7 is a diagram showing interference of light.

FIG. 8 is a diagram showing the conventional process of generating atracking error signal;

FIG. 9 is a diagram showing the prior art;

FIG. 10A is a diagram showing the tracks in an exemplary prior art disk;and

FIG. 10B is a diagram showing the tracks in another exemplary prior artdisk.

BEST MODE FOR CARRYING OUT THE INVENTION

Preferred embodiments of the present invention will be described indetail hereinafter with reference to the drawings.

FIGS. 1 and 2 show an example of a magneto-optical disk to which thepresent invention is applied. The magneto-optical disk D of the presentembodiment is a groove-recording disk in which lands and grooves areformed in the recording layer, and only the grooves are used as tracksfor data recording.

As shown in FIG. 1, the tracks T formed in the recording layer of themagneto-optical disk D extends along a single spiral. The tracks Tinclude a plurality of first sections Ta (indicated by solid lines) anda plurality of second sections Tb (indicated by dashed lines).

As shown in FIG. 2, each of the first sections Ta is a section in whicha plurality of first grooves Ga (portions marked with crosshatching) areformed along the track direction Tc. Each of the second sections Tb is asection in which a plurality of second grooves Gb are formed along thetrack direction Tc. The first and second grooves Ga, Gb have the samewidth in the tracking direction Tg, but have different depths. In thepresent invention, the first and second grooves Ga, Gb may have the samedepth but different widths in the tracking direction Tg, or may differin both depth and width in the tracking direction Tg.

Each of the first and second sections Ta, Tb is made slightly longerthan the length of turn of the track T. This results in the formation ofa plurality of first overlapping portions 11 in each of which theleading end 11 a and trailing end 11 b of a respective first section Taoverlap in the track direction Tc, and a plurality of second overlappingportions 12 in each of which the leading end 12 a and trailing end 12 bof a respective second sections Tb overlap in the track direction Tc, asshown in FIG. 1.

Referring to FIG. 2, a more specific description is given by taking thefirst section Ta (Ta′) extending from the (n1)th track T_(n−1) to thenth track T_(n) for example. The first grooves Ga′, Ga″ corresponding tothe trailing end and leading end, respectively, of the first section Ta(Ta′) are positioned in adjacent tracks and overlap each other in thetrack direction Tc by an appropriate amount s1 (the direction indicatedby the arrow Fr is the direction in which the optical head movesrelative to the magneto-optical disk D). In any other one of the firstsections Ta, similarly, the first grooves Ga at the trailing and leadingends are positioned in adjacent tracks at other locations and partiallyoverlap each other in the track direction Tc.

The second sections Tb are configured in the same manner. Taking, as anexample, the second section Tb (Tb′) extending from the nth track T_(n)to the (n+1)th track T_(n+1), the second grooves Gb′, Gb″ correspondingto the trailing end and leading end, respectively, are positioned inadjacent tracks and overlap each other in the track direction Tc. Thefirst and second overlapping portions 11 and 12 alternate with eachother at every turn of the tracks T, and either one of the first orsecond overlapping portions 11 or 12 is present in every turn of thetracks T.

The first and second grooves Ga, Gb have the same length at least in thesame track T and are arranged at a constant pitch. Portions of thetracks T between the grooves are gaps 13 having the same height as thelands L. The gaps 13 are provided in an orderly manner, as describedbelow.

As shown in FIG. 3, the gaps 13 (13 a) in the nth track Tn are offsetfrom the gaps 13 (13 b) in the preceding (n1)th track T_(n−1) in thetrack direction Tc by ½ the pitch Pa between the preceding track gaps.In this way, the gaps 13 between each two adjacent tracks T in themagneto-optical disk D are thus offset by ½ the arrangement pitch of thegaps 13 in the track T. Consequently, each of the gaps 13 is sandwichedbetween the first and/or second grooves Ga, Gb in the tracking directionTg.

FIG. 4 illustrates an example of a magneto-optical disk device to whichthe present invention is applied.

The magneto-optical disk device A of the present embodiment includes aspindle motor SM for supporting the magneto-optical disk D describedabove and rotating it at high speed, an optical unit 2, and a signalgenerating unit 3.

The optical unit 2 is designed so that laser light emitted from a laserdiode 20 is collimated by a collimator lens 21, then passed through ahalf mirror 22, and caused to enter an objective lens 23 of an opticalhead H. The laser light incident on the objective lens 23 is focused toform a beam spot on the recording layer of the magneto-optical disk D.The light reflected by the recording layer passes again through theobjective lens 23 and is returned to the half mirror 22 where it changesthe propagation direction for entering into the signal generating system3. The optical system 2 is provided with a seek motor M1 for moving theoptical head H towards the position that faces the target track of themagneto-optical disk D, and with a tracking control actuator 24 forcausing the objective lens 23 to track the target track. Though notshown in the drawings, the optical head H is provided with a coil forgenerating a magnetic field that acts on the portion in which the beamspot is formed on the magneto-optical disk D.

The signal generating unit 3 has an optical detector 30, a push-pullsignal generator 31, a filter 32, a sample hold circuit 33, a servocontrol circuit 34, and other circuits described hereinafter.

The optical detector 30 includes, for example, a photoelectricconversion element and has a light-receiving surface 30a for receivingreflected light from the magneto-optical disk D. The light-receivingsurface 30 a is divided into four equal regions Aa˜Ad for receivinglight divisionally in the tracking direction Tg and in the trackdirection Tc. The optical detector 30 outputs signals Sa˜Sdcorresponding to the amount of light (the intensity of light) receivedin the four regions Aa through Ad.

The push-pull signal generator 31 creates a radial push-pull signal(hereinafter referred to as “RPP signal”) and a tangential push-pullsignal (hereinafter referred to as “TPP”) on the basis of the signalsSa˜Sd outputted from the optical detector 30. When the output levels ofthe signals Sa˜Sd are represented as a˜d, the level L_(RPP) of the RPPsignal satisfies the relation: L_(RPP)=(ab) c+d). The RPP signal is asignal corresponding to the intensity distribution, in the trackingdirection Tg, of light reflected from the magneto-optical disk D. Inprinciple, the RPP signal is zero if the tracking is correct.

The level L_(TPP) of the TPP signal satisfies the relation:L_(TPP)=(ac) b+d). The TPP signal corresponds to the intensitydistribution, in the track direction Tc, of light reflected from themagneto-optical disk D. Consequently, the TPP signal is zero when thebeam spot is formed at the longitudinal center of any one of the firstand second grooves Ga, Gb of the magneto-optical disk D. When the beamspot is formed in any gap 13 or in the vicinity thereof, an interferencelight pattern such as described with reference to FIG. 7 is formed alongthe track direction Tc, whereby the TPP signal fluctuates.

FIG. 5A shows a specific example of the waveform of the TPP signal whenthe beam spot is formed in a gap 13 or in the vicinity thereof. As shownin FIG. 5C, these data show the relation between the distance from thecenter C of the gap 13 and the output level (relative value) of the TPPsignal when the beam spot is formed at that distance in the cases wherethe width s2 of the gap 13 is set to a plurality of predeterminedvalues. The track-to-track pitch s3 was 0.24 μm; the groove width s4 was0.16 μm; and the total length s5 of one gap 13 and one first groove Gawas 10 μm. The NA of the optical head H was 0.85, and the wavelength ofthe laser beam was 405 nm. As is apparent from FIG. 5A, the TPP signalswings to the negative side when the beam spot enters the gap 13,becomes zero when the beam spot reaches the center of the gap 13, swingsto the positive side, and then returns to zero.

As shown in FIG. 4, the TPP signal is inputted to an amplitude/zeropoint detection circuit 37. The amplitude/zero point detection circuit37 detects the fluctuation of, and the zero point in the fluctuation of,a TPP signal such as described with reference to FIG. 5A, and outputs apredetermined pulse reference signal S1 when the zero point is detected.This reference signal S1 is inputted to the sample hold circuit 33 andthe PLL circuit 38. The PLL circuit 38 generates a clock signal CLK onthe basis of the reference signal S1, and the clock signal CLK isinputted to a read/write control circuit 39. The clock signal CLK isused as a timing signal for the read/write control circuit 39 incontrolling the actuation circuit 40 which drives the laser diode 20.

The RPP signal is inputted to both the filter 32 and the sample holdcircuit 33. The filter 32 performs the role of removing thehigh-frequency noise component from the RPP signal. As alreadydescribed, the RPP signal is zero if the tracking is correct, but canbecome mixed with a high-frequency noise component when the beam spot isformed in the gap 13. The filter 32 creates a first tracking errorsignal from the RPP signal by removing the noise component from the RPPsignal. The first tracking error signal corresponds to the trackingerror signal in the conventional technique, and is inputted to the servocontrol circuit 34 and a zero point detection circuit 35.

The zero point detection circuit 35 is a circuit for detecting the zeropoint of the first tracking error signal, and the number of detectionsis counted by a track counter 36. During a seek operation, the countnumber corresponds to the number of tracks T traversed by the opticalhead H, or to the distance traveled by the optical head H. The countnumber signal is also inputted to the servo control circuit 34.

The sample hold circuit 33 is a circuit for sampling the RPP signal withthe timing at which the zero point is detected in the TPP signal by theamplitude/zero point detection circuit 37, thereby creating a secondtracking error signal. This aspect will be described in detailhereinafter.

FIG. 5B shows a specific example of the waveform of the RPP signal. Theconditions whereby this RPP signal is obtained are the same as forobtaining the TPP signal shown in FIG. 5A. However, the level of the RPPsignal is zero or consists solely of a high-frequency noise componentwhen the beam spot is aligned in the center of the track, so the RPPsignal shown in FIG. 5B is the signal obtained when the beam spot isformed in a position that is offset in the tracking direction Tg by anappropriate distance from the track center, as indicated by the arrow N1in FIG. 5C.

When the zero point indicated by the symbol N2 in FIG. 5A is detected,the sample hold circuit 33 samples an RPP signal such as the one shownin FIG. 5B and outputs the sampled signal. This signal consists of asecond tracking error signal. Therefore, this second tracking errorsignal carries the nuance of a tracking error signal in the center C ofthe gap 13.

The servo control circuit 34 is configured so as to perform actuationcontrol of the actuator 24 and the seek motor M1 to perform trackingcontrol and seek operation on the basis of the first and second trackingerror signals. The servo control circuit 34 is configured so thattracking control is performed on the basis of the second tracking errorsignal when processing is executed for the writing or reading of data toor from the magneto-optical disk D, but the tracking control isperformed on the basis of the first tracking error signal during idletime in which such writing or reading is not executed.

The operation of the magneto-optical disk device A will next bedescribed.

First, tracking control is executed on the basis of a first trackingerror signal during idle time. During this idle time, assuming that thebeam spot formed on the first section Ta (Ta′) of the nth track Tn inFIG. 2, for example, enters the first groove Ga′ at the trailing end ofthe first section Ta (Ta′), the second groove Gb′ is present on one sideof the first groove Ga′ in the tracking direction Tg, whereas the firstgroove Ga″ is present in the opposite side. Thus, the apparent spatialfrequency becomes ⅓, and a first tracking error signal can appropriatelybe obtained at this time as well.

The first tracking error signal obtained at this time is such that it isformed as if the track center is located between the two first groovesGa′ and Ga″. Therefore, if tracking control is performed on the basis ofthis first tracking error signal, the beam spot shifts from the firstgroove Ga′ to the first groove Ga″ for subsequently causing the beamspot to move along the first section Ta (Ta′) on the (n1)th trackT_(n−1), as indicated by the arrow N3 in FIG. 2. As a result, the beamspot is caused to circulate only along the first section Ta (Ta′) duringidling.

A similar function can be obtained in the same manner for the secondsection Tb as well. For example, a beam spot formed on the secondsection Tb (Tb′) of the (n+1)th track T_(n+1) can be caused to proceedas indicated by the arrow N4 in the second overlapping portion 12 andcan be caused to circulate only along the second section Tb (Tb′).

Thus, according to the present embodiment, the beam spot can be causedto circulate for facilitation of control simply by performing trackingcontrol on the basis of the first tracking error signal irrespective ofthe fact that the track T of the magneto-optical disk D has a spiralshape. Either a first or a second overlapping portion 11 or 12 isprovided in every turn of the tracks T, so that circulation of the beamspot during idling as described above can be performed in any of thetracks, which is better for smoothly performing the above-mentionedcontrol.

The first tracking error signal can also be prevented from sharplychanging in the portion in which the first and second sections Ta and Tbchange over, by providing the first and second overlapping portions 11,12 in the tracks T. Specifically, according to the prior art shown inFIG. 10B, the location at which the first section changes over to thesecond section in one track is flanked by a point of shift from thesecond section to the first section in the adjacent tracks. By contrast,the inventive magneto-optical disk D does not have this type ofconfiguration, and a point of shift from the first section Ta to thesecond section Tb in one track is flanked by two adjacent tracks but anaccompanying shift from the second section Tb to the first section Taonly in one of the two tracks. Therefore, the amount of variance in thefirst tracking error signal according to the present embodiment can bereduced compared to that in the tracking error signal according to theabove-described prior art. Consequently, as compared to the prior art,there is a reduced risk of erroneously counting a sudden change in thetracking error signal as a track count can be reduced when the trackcounter 36 counts the number of tracks by counting the number of zeropoint detections in the first tracking error signal during a seekoperation. The present magneto-optical disk device A tends to give alower count with respect to the number of tracks, but a lower count oftracks provides an easier correction of seek operation than a highercount.

On the other hand, when data are written to or read from themagneto-optical disk D, tracking control is executed on the basis of thesecond tracking error signal. As previously described, the secondtracking error signal is a kind of signal that corresponds to a trackingerror signal at the center of the gap 13, and is therefore differentfrom the first tracking error signal in that it is not affected by thefirst and second overlapping portions 11, 12. Since the plurality ofgaps 13 are provided at a constant interval in any turn of the tracks T,the second tracking error signal can be consistently obtained at aconstant interval. Consequently, it is possible to perform highlyaccurate tracking control on the basis of the second tracking errorsignal, and processing for writing or reading of data can beappropriately performed.

The clock signal CLK used when data are written or read is created onthe basis of the plurality of gaps 13, and a correct clock signal CLKcan be obtained because the interval between the plurality of gaps 13 isconstant at least in the same track. In contrast to the presentembodiment, in the case where there is no consistency with respect tothe positions of the gaps 13 among the plurality of tracks Ts forexample, a mismatch in the phase relation between the PLL and the gaps13 occurs and resynchronization must be performed when a so-called trackshift is performed in a seek operation for example. In themagneto-optical disk D, however, the amount of the positional offsetbetween gaps 13 in two adjacent tracks T is designed to be ½ the pitchPa of the gaps 13 in one of these two tracks T, as described withreference to FIG. 3. Therefore, if the above-mentioned amount of thepositional offset is used as a reference clock interval, mismatch in thephase relation between the PLL and the gaps 13 can be avoided even whena so-called track jump is performed. It is therefore possible toeliminate the need for performing resynchronization every time aso-called track shift is performed. It should be noted that such anadvantage is not limited to the case where the amount of the positionaloffset described above is set to ½ the pitch Pa of the gaps 13, and mayalso be obtained when the offset is ⅓, ¼, or another fraction of 1 withan integer denominator of 2 or more.

The present invention is not limited by the foregoing embodiment. Thespecific structure of the optical recording medium and of the componentsof the optical information processing apparatus according to the presentinvention is also subject to various design modifications.

FIG. 6 shows another embodiment of the present invention. Each of thesecond overlapping portions 12 in this embodiment is such that thegroove Gb1 at the trailing end of the second section Tb does not overlapthe groove Gb2 at the leading end of the second section Tb. Instead, itoverlaps other grooves Gb3 and Gb4. The beam spot can be caused to moveas indicated by the arrow N5 in the same figure, and the functionintended for by the present invention can be obtained in this structureas well. It is apparent from this embodiment that the first and secondoverlapping portions referred to in the present invention need not bedesigned such that the trailing ends of the first and second sectionsoverlap the leading ends of the respective sections, and may overlap anintermediate portion instead of the leading end. The specific length ofthe overlap is also not limited.

The present invention may also be applied to a type of optical recordingmedium that differs from a groove recording format; for example, aread-only optical disk provided with rows of pits as tracks. It ispossible with this read-only optical disk to obtain clock information byforming the beam spot in the pits, and the structure of the signalgenerating system differs in this respect from the magneto-optical diskdevice A described above.

1. An optical recording medium comprising: a recording layer having asurface which is indented in cross section extending in a trackingdirection, said surface being formed with tracks extending along aspiral; the tracks including a plurality of first sections and aplurality of second sections, the first sections differing from thesecond sections with respect to at least one of width and depth, thefirst sections alternating with the second sections in a manner suchthat a trailing end of one section is connected to a leading end ofanother section; a first overlapping portion in which the trailing endand another portion of each first section overlap each other in thetracking direction, said trailing end and said another portion of saideach first section being located in adjacent tracks; and a secondoverlapping portion in which the trailing end and another portion ofeach second section overlap each other in the tracking direction, saidtrailing end and said another portion of said each second section beinglocated in adjacent tracks.
 2. The optical recording medium according toclaim 1, wherein the recording layer comprises grooves and landsalternating with the grooves in the tracking direction.
 3. The opticalrecording medium according to claim 2, which is a groove-recording diskfor writing data in the grooves.
 4. The optical recording mediumaccording to claim 1, which is a read-only disk wherein the tracks areformed with a plurality of pits for data.
 5. The optical recordingmedium according to claim 1, wherein at least one shift point betweenthe first section and the second section is provided in every turn ofthe tracks.
 6. The optical recording medium according to claim 2,further comprising a plurality of gaps for creating a thicknesswiseheight difference in the tracks, the gaps positioned in each track beingoffset from the gaps positioned in an adjacent track in the trackdirection.
 7. The optical recording medium according to claim 6, whereinthe gaps are formed by spacing the grooves from each other in the trackdirection.
 8. The optical recording medium according to claim 7, whereinthe gaps are arranged at a constant pitch at least in a same track. 9.The optical recording medium according to claim 8, wherein the amount ofthe offset in the track direction between the gaps in adjacent tracks isset at 1/N (where N is an integer of no less than 2) of the gap pitch inone of the tracks.
 10. An optical information processing apparatuscomprising: an optical head disposed in facing relationship to anoptical recording medium for emitting a beam of light to the opticalrecording medium and for receiving the light reflected from the opticalrecording medium; an optical detector for receiving the reflected lightfrom the optical recording medium via the optical head to output asignal corresponding to an intensity distribution of the reflectedlight; and a push-pull signal generator for generating a radialpush-pull signal and a tangential push-pull signal in dependence on anintensity difference of the reflected light in a tracking direction andin a track direction on the basis of the signal outputted from theoptical detector; a first signal processor for generating a firsttracking error signal by removing a noise component from the radialpush-pull signal; a second signal processor for generating a secondtracking error signal by sampling and holding the radial push-pullsignal when a predetermined change has occurred in the tangentialpush-pull signal; and a controller for performing tracking control onthe basis of the first tracking error signal during idle time in whichdata are neither written in nor read from the optical recording mediumwhile performing tracking control on the basis of the second trackingerror signal when data are written or read.
 11. The optical informationprocessing apparatus according to claim 10, wherein the first signalprocessor is a filter for removing a high-frequency component of no lessthan a predetermined minimum frequency.
 12. The optical informationprocessing apparatus according to claim 10, further comprising a trackcounter for counting the tracks during a seek operation by detecting andcounting zero points of the first tracking error signal.
 13. The opticalinformation processing apparatus according to claim 10, wherein thesecond signal processor comprises a detection circuit for detectingamplitude and zero point of the tangential push-pull signal; and asample hold circuit for sampling the radial push-pull signal when apredetermined amplitude and the zero point are detected by the detectioncircuit.
 14. The optical information processing apparatus according toclaim 13, further comprising a clock signal generator for generating aclock signal in synchronization with a timing at which the zero point isdetected by the detection circuit.